Molds for injection molding of plastics have hitherto been made of steels, zinc alloys, aluminum alloys, and the like. Steels have sufficient mechanical strengths for molds, and steel molds are now in use for large-quantity production. They, however, have the disadvantages of high cost and long time period required for the mold making. In contrast, the alloys based on zinc or aluminum can be made into molds at lower costs and in shorter periods of time. Especially, zinc alloys typified with Zn-Al-Cu-Mg system are advantageous in that they can be formed into molds by simpler casting methods. However, inadequate mechanical strengths restrict the use of the molds of zinc and aluminum alloys to trial manufacture and small-quantity production.
Medium-quantity production is assuming increasing importance due to the recent trend towards shorter intervals between model changes in automobiles, office automation equipment, and the like. When steel is used for medium-quantity production of their plastic parts, a problem arises in that the mold price forms a considerable proportion of the overall manufacturing cost. Nevertheless, steel has to be employed because the existing zinc and aluminum alloys do not possess the necessary mechanical strengths.
Among steels, zinc alloys, and aluminum alloys, the cheapest to make into molds are zinc alloys, owing to the simplest casting procedure involved. Generally, in the manufacture of a mold by casting, the material is required to exhibit outstanding castability under the conditions that are encountered by large-size and slow cooled castings. This requirement is met by the zinc alloys commercially available today. Thus, if a zinc alloy can attain improved mechanical strength while maintaining its excellent castability, it will serve as the best material for medium-quantity production molds.
It is an object of this invention to have a zinc based alloy having a high strength and good castability which is suitable as a material for molds for injection molding of plastics or as die casting products.